1. Field of the Invention
This invention relates to ceramic materials and more particularly to ceramic facing materials for gas turbine, outer air seals.
2. Description of the Prior Art
The construction of outer air seals for gas turbine engines has received significant attention in the past and effective embodiments of such seals are continually sought. In an axial flow gas turbine engine, rows of rotor blades in both the compressor and turbine sections of the engine extend radially outwardly on the rotor assembly across a flowpath for working medium gases. An outer air seal which is affixed to the stator assembly circumscribes the tips of the blades and each blade row and inhibits the leakage of working medium gases over the tips of the blades. Each turbine outer air seal is conventionally formed of a plurality of seal segments disposed in end to end relationship about the engine. The tip opposing surfaces of each segment are commonly formed of an abradable material to enable a closely toleranced, initial condition without destructive interference from the blade tips in transient modes. Representative abradable seal lands and methods of manufacture are illustrated in U.S. Pat. Nos. 3,817,719 to Schilke et al entitled "High Temperature Abradable Material and Method of Preparing the Same"; 3,879,831 to Rigney et al entitled "Nickel Base High Temperature Abradable Material" ; 3,918,925 to McComas entitled "Abradable Seal"; and 3,936,656 to Middleton et al entitled "Method of Affixing an Abradable Metallic Fiber Material to a Metal Substrate".
Notwithstanding the availability of the aforementioned materials and designs, manufacturers of gas turbine components continue to search for yet improved abradable material constructions having adequate durability in hostile environments. Particularly, within the turbine sections of engines where seal materials are exposed to local temperatures which may exceed twenty-five hundred degrees Fahrenheit (2500.degree. F.), material and structure selections having adequate durability are limited. Ceramic faced seals are of prime interest for these components.
Ceramic materials in general are known to be effective thermal insulators in gas turbine environments and are currently utilized as coating materials for metallic substrates in high temperature environments. As long as the coating materials remain intact, such ceramics prevent unacceptable deterioration of the metallic forms to which they are adhered. Metallic and ceramic materials are not wholly compatible, however, as a large difference in coefficients of thermal expansion between the two material types makes long term adherence of the ceramic to the metal difficult. Typically, subsequent thermal cycling of the finished part in the intended environment causes cracking and spalling of the ceramic from the metal. Such problems are particularly severe where depths of coating in excess of a very few thousandths of an inch are desired.
One ceramic faced seal structure which is adapted to accommodate differences in coefficients of thermal expansion between the ceramic facing material and an underlying metallic substrate is disclosed in U.S. Pat. No. 4,109,031 to Marscher entitled "Stress Relief of Metal-Ceramic Gas Turbine Seals". Graded layers of material in which the relative amounts of metal and ceramic are varied from one hundred percent (100%) metal at the metal interface to one hundred percent (100%) ceramic at the ceramic interface are applied to the metal substrate.
Another type of ceramic faced seal structure is discussed in a paper delivered at the 1976 Joint Fall Meeting of the Basic Science, Electronics and Nuclear Divisions of the American Ceramic Society entitled "Bonding Ceramic Materials to Metallic Substrates for High-Temperature, Low-Weight Applications" and in NASA Technical Memorandum, NASA TM-73852, entitled "Preliminary Study of Cyclic Thermal Shock Resistance of Plasma-Sprayed Zirconium Oxide Turbine Outer Air Seal Shrouds". In accordance with the disclosed systems, a mat of sintered wires joins a ceramic layer to an underlying metallic substrate. The wires form a compliant layer which is capable of accommodating differential thermal expansion between substrate and ceramic layers. In the former structure an alumina (Al.sub.2 O.sub.3) ceramic material is applied directly to the wire mat. In the latter structure a zirconium oxide (ZrO.sub.2) ceramic material is applied over a bond coat of three to five thousandths of an inch (0.003-0.005 in.) to a wire mat and screen.
Although the structures discussed above are known to be highly desirable if adequate ceramic durability can be achieved, the structures have yet to achieve full potential, particularly in hostile environment applications. Significant research into the mechanical properties of the desired ceramic material continues in the search for durable structures.